Document 1Unexamined Japanese patent 2000-146908 (U.S. Pat.No. 6200443)Document 2U.S. Pat. No. 5650,054Document 3U.S. Pat. No. 4820,386Document 4Unexamined Japanese patent 2002-350393
Documents 1 through 3 disclose structures of proton conductor gas sensors having water reservoirs. In document 1, a sensing electrode and a counter electrode are provided on both sides of a proton conductive membrane to form a membrane electrode assembly (MEA), and the MEA is sandwiched with hydrophobic and porous electro-conductive carbon sheets. The upper and lower carbon sheets are interposed by a pair of metal plates having openings, and they are fixed in a metal can with a water reservoir. Water vapor from the water reservoir moves through the opening of the lower metal plate and, via the hydrophobic carbon sheet, reaches the counter electrode. The ambient atmosphere diffuses through the opening of the upper metal plate to the sensing electrode. Thus, necessary electrode reactions take place at both the sensing electrode and the counter electrode, and the electromotive force, the current value, etc. enable the detection of a gas in the ambient atmosphere.
If liquid water, however, is contained in the water reservoir, for example, with a sudden rise in the water reservoir temperature, the air in the water reservoir swells to extrude water. A countermeasure against this is to gelate water as taught in document 3. If a water absorbing polymer such as polyacrylic one is used for the reservoir, it may contaminate the MEA because it contains metal ions such as Na+. Hence the present inventors considered using a natural polymer as a gelling agent but found that such a gel is solated at elevated temperature to affect the sensor output. Accordingly, a gel having a low content of metal ions and not being solated at elevated temperature is required. Moreover, to extend the service life of the water reservoir, a gelling agent that can hold a large amount of water is desirable.
In document 2 a lower part of a metal can is used as a water reservoir, a washer is arranged above the lower part of the metal can, an MEA is placed on the washer, and a cap is arranged on the upper side of the MEA. As a result, the metal can is connected to the counter electrode, and the cap is connected to the sensing electrode.